The present invention relates to a vinyl chloride resin composition having excellent weather resistance and impact resistance.
It is generally known that shaped articles of vinyl chloride resin have poor impact resistance. To improve the impact resistance of vinyl chloride resin, a variety of methods have been proposed.
The most representative modifier used for this purpose today is MBS resin which is prepared by graft-polymerizing methyl methacrylate and styrene, and, optionally, acrylonitrile, on a butadiene rubber polymer. However, if this MBS resin is added to a vinyl chloride resin, the resulting blend is improved in impact resistance but poor in weather resistance and if a shaped article made of such blend is used outdoors, it suffers a serious decrease in impact strength. This is the reason why MBS resin has found only limited application.
This decrease in weather resistance is mainly attributed to the ultraviolet light degradation of the butadiene unit of MBS resin. To improve the weather resistance of a modifier to be added to a vinyl chloride resin and to impart adequate impact resistance to the resulting vinyl chloride resin composition, there has been proposed a method which comprises graft-polymerizing an alkyl methacrylate, and, optionally, an aromatic vinyl compound and an unsaturated nitrile compound, onto a cross-linked acrylate rubber-like polymer containing no double bond which is composed of an alkyl acrylate and a cross-linking agent (Japanese Examined Patent Publication No. 51-28117).
When the graft copolymer thus obtained is employed as a modifier, shaped articles made of the resulting vinyl chloride resin composition have superior weather resistance. Therefore, the shaped articles have recently been used on a commercial basis in applications demanding long-term weather resistance such as window frames.
However, while the use of this graft copolymer yields a satisfactory weather resistance in shaped articles, the impact resistance of the articles is not as good as desired.
It is known that the above problem can be obviated by increasing the proportion of the rubber-like polymer in the graft copolymer. However, whereas an increased proportion of the rubber-like polymer results in greater impact strength, the agglomeration of graft copolymer particles in the course of their precipitation from the aqueous dispersion of the graft copolymer is increased to the extent that the powder state of the graft copolymer is seriously affected and it is known that, in extreme cases, the graft copolymer is available only in the form of rubbery blocks.
Thus, from the standpoint of commercial production of the graft copolymer, the usable proportion of the rubber-like polymer in the graft-copolymer has been limited by this requirement that the resulting graft copolymer must be in a good powder state.
As described above, when a graft copolymer made from a rubber-like polymer based on alkyl acrylates is used in a vinyl chloride resin composition, there is encountered the problem that increasing the proportion of the rubber-like polymer in order to improve the impact resistance of final shaped articles leads to an increased degree of agglomeration of graft copolymer particles precipitated from an aqueous dispersion so that the powder state of the obtained copolymer is seriously affected and it is difficult to obtain a copolymer in a good powder state. If, to overcome this problem, the molecular weight of the graft phase to be mentioned later is increased, there is the problem of "die swelling", i.e. the expansion of the graft copolymer-vinyl chloride resin composition which takes place at an extrusion die in the extrusion molding of the composition.
It is an object of the present invention is to solve the above problem in the production of the graft copolymer, which would otherwise occur as the result of increasing the proportion of the rubber-like polymer, to eliminate the above-mentioned drawback of "die swelling" in the extrusion molding of the resulting vinyl chloride resin composition and to impart excellent impact resistance and excellent weather resistance to shaped articles.
This and other objects of the invention will become apparent from the description hereinafter.